The other day I'm on a wide, well-maintained, two-lane road with big shoulders on each side, the speed limit is 55 mph, and it's a nice day outside, perfect weather, and the asshole in front of me is going 35 mph. And it was a no passing zone due to turns and stuff.
Now my question is, why the hell do people go under the speed limit, when there are no adverse conditions, or situations that would cause them to do so? It's just infuriating.

You should try driving this way in a less advanced infrastructured country like me where we have some like 320 km of highway rest is just two-lane roads or extended express two-way roads. If you get cought by a guy going under the speed limit or at the speed limit good luck making where you have to be on time. I overspeed about 90% the time, due the fact that to make 300km I have to drive about 5.5-6h. And ofc overtaking when possible not that much via sign for "no passing" zone but by common sense and knowing the distance, time needed and such.

Suck tho, since if you don't get used to it and learn to judge fast you end up in a car crash, hence why we do have a quite high death via car accidents.

ps: A good advantage for a SUV or any high driver position car in this scenario of driving is that you most of the time can see further and past the car in front of you, making overtaking more easy. Just like semi truck drivers can see about two curbs ahead of you since they are way way higher.